Whatever Happened to Acid Rain?

The possible effects of acid deposition on crops, forests, streams, stone structures, and health have been a concern throughout North America and Europe since the phenomenon was first widely publicized in the 1970s. Despite dire forecasts of widespread fish kills and the loss of forests, these impacts have not occurred. The effects of acid deposition on aquatic and forest ecosystems in both the U.S. and Europe are still widely debated. To date, the aquatic impacts of acid deposition appear to be confined to certain surface waters in watersheds that have soils with very low ability to buffer acid inputs. In the upper reaches of some streams and in some lakes in glaciated regions, populations of trout and other fish have been reduced or are periodically stressed by acidic storm events.

Effects of acid deposition on forest ecosystems in both the U.S. and Europe have been widely debated. Forests remain intact without widespread declines in growth, with the exception of some high elevation spruce-fir forests. According to reports from the National Acid Precipitation Assessment Program, many scientists now believe that in eastern North America, high elevation spruce-fir forests have been exposed to additional stress from acid deposition, resulting from soil acidification, loss of base cationic nutrients, and/or direct impacts of acidic fog. However, quantifying these effects and separating natural events from human-caused stresses has proved very difficult and remains controversial.

Acidic deposition occurs largely as a result of the fallout of sulfuric acid, nitric acid, and ammonium (NH4) in precipitation, dry deposition, and cloud water (fog) deposition. The primary source of sulfuric acid is sulfur dioxide (SO2) present in air emissions from coal-fired power plants. Nitric acid is derived from nitrogen oxides (NOx), which are emitted by a variety of sources, including vehicles that contribute about one-third of U.S. NOx emissions and power plants that account for another one-third. Ammonium also contributes to nitrogen (N) deposition, which can ultimately acidify soils and surface waters. The primary source of NH4 is ammonia (NH3) emitted by agricultural operations, especially fertilizers and livestock feedlots.

Reductions in SO2 Emissions and Acid Deposition

Electric utilities in the eastern U.S. have been very successful in their efforts to reduce SO2 emissions from coal-fired power plants. Air concentrations of SO2 declined markedly from 1979 to 2002. For example, average 24-hour, and average 3-hour SO2 levels declined by more than 50% in the Tennessee Valley Region over this 20-year period (see TVA’s report “How clean is the air? Air Quality in the Tennessee Valley Region”).

The decline of SO2 emissions in the eastern U.S. over the past 16 years corresponds to a marked decline in the amount of sulfate and a moderate decline in the total acidity of precipitation. Data from the National Acidic Deposition Program indicate that 3-year average sulfate concentrations in precipitation during the years 1984-1986 for much of the Midwest (Indiana, Ohio, Pennsylvania and portions of West Virginia and New York) were greater than 3 milligrams/liter (mg/L) (Figure 1). By 2000-2002, sulfate levels throughout the Midwest had declined to 1.5 to 2.0 mg/L (Figure 2).

Similarly, across most of the eastern half of Tennessee, precipitation sulfate averages had been greater than 2 mg/L, whereas today the range (0.75 to 2.0 mg/L) across the entire state is below that level. The NADP has six air quality monitoring sites in Tennessee and western North Carolina. Data gathered from these stations show both rainfall-sulfate and acidity levels have declined on average about 20% since the late 1980s.

National Atmospheric Deposition Program / National Trends Netowork

Figure 1. Average sulfate ion concentrations in deposition across the U.S. during the period 1984-86 (as indicated in the boxed dates in the bottom scale). (Data from National Acid Deposition Program web site, 2004.)

National Atmospheric Deposition Program / National Trends Netowork

Figure 2. Average sulfate ion concentrations in deposition across the U.S. during the period 2000-2002 (as indicated in the boxed dates in the bottom scale). (Data from National Acid Deposition Program web site, 2004.)

Precipitation acidity has declined noticeably less than have SO2 emissions. In the TVA region, SO2 emissions have declined about 50% during the same period that rainfall acidity has dropped only 20%. The primary reason for this discrepancy is that nitrogen emissions have not declined significantly since 1980. NOx reductions from power plants under Title IV acid rain controls have been offset by increases in NOx from vehicles and increases in NH3 emissions from livestock operations. As a consequence, total N levels (including NOx and NH4) in precipitation have remained relatively constant over the same period.

Predictions of Emissions and Deposition

The Southern Appalachian Mountains Initiative (SAMI) has used mathematical models to predict future changes in total sulfur (S) and nitrogen emissions and deposition through 2040 for several emissions control strategies. Under SAMI’s reference strategy, called “On the Way” (OTW), models assume both that regulations promulgated prior to 1997 would remain in place (chiefly Title IV of the 1990 Clean Air Act Amendments) and that new regulations would be implemented. These new regulations include Tier II highway vehicle and fuel rules and the NOx State Implementation Plan. Under this reference strategy, the models predict SO2 emissions will drop by another 50% by 2040. While reductions in NOx emissions are predicted to decline by another 25% by 2010 and by an additional 5% by 2050, no change in total nitrogen deposition is predicted. This anomaly is the result of predicted increases in ammonia emissions from agriculture and other sources during the same time period.

Current Conditions and Predictions for Streams and Forests

I. Streams The current conditions of streams in one of the most acid-sensitive regions of the United States—the southern Appalachians—were assessed for the 1991-1995 period by SAMI. The best indicator of stream acidity, and of a stream’s capability for supporting fish and other aquatic life, is its acid neutralizing capacity (ANC). According to SAMI’s survey of southern Appalachian streams, 2% of stream lengths are chronically acidic, 1% are episodically acidic, and 4% are in the moderately acidic category. In other words, within the southern Appalachian region, about 93% of all stream lengths are not particularly sensitive to further acidification. Almost all the streams that are currently acidic or sensitive are headwater streams above 3000 feet in elevation.

Models of the effects of future reductions in emissions and deposition of S and N predict rather small changes in stream ANC conditions through 2040. The good news is, that when compared to predictions under a scenario of “No Change” in emissions from the 1991-95 level, the “On the Way” reductions in S and N emissions should prevent 30 to 40% of the most sensitive streams from becoming chronically acidic. More stringent controls on SO2, NOx, and NH4 emissions would achieve only a slight improvement. From another perspective, however, the overall number of streams with low ANC is not expected to decrease through 2040, even with the reductions in emissions such as in the “On the Way” scenario. Only prevention of further deterioration in stream quality with regard to ANC is predicted.

II. Forests. The future condition of high-elevation forests in the southern Appalachians is difficult to predict. Our knowledge of the linkages between soil conditions and forest health remains limited because of the difficulty of separating damage to forests resulting from pests and weather extremes from any damage due to acidic deposition. Models used in the SAMI report indicate that regardless of reductions in atmospheric S and N emissions, indices of soil acidity will in most cases, remain approximately the same or worsen slightly over the coming decades. We still cannot accurately predict what impact these slight increases in soil acidification will have on the health of high elevation forest ecosystems. While we should not expect soils to recover from past acidification in the near future, emission reductions should prevent or slow the rate of soil deterioration.

Conclusions

Major reductions in power plant emissions of SO2, in rainfall concentrations of sulfate, and in the deposition of total acidity have been achieved in eastern North America in response to concerns that surfaced in the 1970’s and 1980s about the impacts of acid rain on ecosystems.
Certain limited areas in eastern North America have surface waters and forests that are currently highly acidic. Predictive models tell us that prevention of further deterioration of these systems is the most realistic goal over the next few decades.
Further reductions in precipitation acidity will require reductions in the N contributions from agricultural NH3 sources and vehicle NOx emissions.
Reductions in SO2 and NOx emissions to address ozone, fine particulate matter, and visibility should simultaneously benefit ecosystems sensitive to acid deposition.

Information Contacts:

For more information on this and other air quality issues, please contact:

L. Suzanne Fisher, 865-632-1451
William J. Parkhurst, 256 386-2793
Frances P. Weatherford, 256 386-2344

If you would like additional information on important air quality topics, please contact Jeanie Ashe by telephone (256-386-2033), E-mail (jbashe@tva.gov), facsimile (256-386-2499), or TVA mail at CEB 2A-M, Muscle Shoals, Alabama 35662.

Last updated on 5-2004.